1. Field of the Invention
The present invention relates to digital image reading apparatuses such as digital copying machines or image scanners, and more particularly to a digital image reading apparatus including a stepping motor.
2. Description of the Related Art
In a conventional digital image reading apparatus reading a document image and transferring the image data to an external apparatus such as a host computer via an interface such as small computer system interface (SCSI), a document reading rate is determined to be a unique value by preset reading density. A rate of transferring the image data to the external apparatus receiving the image data (hereinafter referred to an image data transfer rate) is fixed by a data processing rate of the external apparatus. Therefore, in the case of transferring a certain amount of image data determined by reading density and a reading area from the digital image reading apparatus to the external apparatus, the image data is transferred to the external apparatus without a delay if the image data transfer rate is higher than the document reading rate. However, if the image data transfer rate is lower than the document reading rate, the image data is temporarily stored in memory housed in the digital image reading apparatus, and the stored image data is transferred in accordance with the image data transfer rate.
In general, the transfer of image data from the digital image reading apparatus to the external apparatus is performed in the following procedure. In step 1, image data read by a reading unit is stored in image memory of a DRAM. In step 2, the image data is transferred from the image memory of the DRAM to the external apparatus. The steps 1 and 2 are performed in parallel. However, there is a difference between a rate of storing the image data into the image memory (hereinafter referred to as an image data storing rate) and the image data transfer rate. Therefore, the image data is gradually accumulated in the image memory if the image data transfer rate is low or the reading of the image data is performed with high density. Since the storage capacity of the image memory is limited, the reading of the image data should be suspended at a certain point.
As a scanning system motor or a document conveying system motor of the digital image reading apparatus, a stepping motor is frequently employed. In controlling the stepping motor, a speed control operation of gradually decreasing the speed of the stepping motor is performed since the stepping motor is characteristically prevented from making an instant stop. At a point when room is generated in the storage capacity of the image memory as the image data is transferred to the external apparatus after the suspension, the stepping motor is driven again by canceling the suspension so that the reading operation is resumed. In resuming the reading operation, a speed control operation of gradually increasing the speed of the stepping motor is performed since the stepping motor is characteristically prevented from being driven instantly.
However, if the image data storing rate is relatively higher than the image data transfer rate, the above-describe speed control operations are sometimes repeated a number of times in a single reading operation. Obviously, a better image quality can be obtained by a reading operation at a constant rate than by a reading operation including the above-described speed control operations. Therefore, frequent occurrence of such a situation may deteriorate the image quality (quality of the read image) to a considerable extent. Indeed, an increase in the storage capacity of the image memory can lower the occurrence frequency of the above-described speed control operations, but at the same time, incurs rising costs.
Therefore, in a reading operation expecting a large data transfer amount of image data, such as a color or multivalue reading operation, a reading rate is preset to be lower than that of a binary reading operation so that the data transfer amount becomes equal to or approaches a read data amount.
However, in the reading operation expecting a large data transfer amount of image data, such as a color or multivalue reading operation, the reading rate is preset to be lower than that of the binary reading operation irrespective of the total image data amount of read image. Therefore, a reading time becomes longer, thus lowering the performance of the apparatus.